Lubricant oils have been used for internal combustion engines, automatic and manual transmissions, final drives, power steerings, shock absorbers, and gears, for their smooth operation. In internal combustion engines, in particular, lubricant oils have been used to lubricate piston rings, cylinder liners, bearings for crank shafts and connecting rods, valve train mechanisms including cams and valve lifters, and other sliding members. In addition to the lubricating purposes above described, they are also used for cooling engines, cleaning and dispersing combustion products, and preventing rust and corrosion.
As described above, lubricant oils for internal combustion engines are required to exhibit a variety of functions. These requirements are becoming even more sever as the engines become more functional, produce higher power and are operated under more sever conditions. In order to satisfy these requirements, lubricant base oils for internal combustion engines are incorporated with a variety of additives, such as ashless dispersants, metallic detergents, antiwear agents, friction reducing agents and antioxidants.
Combustion gases produced by an internal combustion engine partly leak into the crank case as blow-by gases through a space between the gaps of piston rings. NO.sub.x gases contained in the combustion gases at a fairly high proportion can deteriorate a lubricant oil in the internal combustion engine, in a concerted manner with oxygen present in the blow-by gases. Lean combustion engines and direct injection engines are now being massively used, to improve fuel economy. These engines are quipped with a 3-element catalytic system to occlude/reduce NO.sub.x or with exhaust gas recirculation (EGR) system, to abate NO.sub.x emissions. A three-element catalyst is known to be poisoned by sulfur, and it is necessary, when the catalytic system is adopted, to control the sulfur poisoning resulting from evaporation of the engine oil. It is also necessary, when an EGR system is adopted, to control deposits at the intake valve and contamination of the EGR control valve with the engine oil components, resulting from inflow of the engine oil into the EGR system.
An engine oil for internal combustion engines, in particular lean-combustion engines, is required to be low in volatility and difficult to be deposited even when it is evaporated to flow into the EGR system. In other words, it is required to be high in oxidation stability. Deposits can be also formed by sludge in the oil, resulting from oxidation and deterioration of the oil by NO.sub.x present in the blow-by gases, and the oil is required to control formation of such sludge.
A variety of additives have been proposed to improve oxidation stability and serviceability of engine oils for internal combustion engines. These engine oils include those incorporated with calcium phenate, magnesium sulfonate and alkenyl succinimide (Japanese Patent Publication No. 3-29839) to agglomerate solid impurities, diesel engine oils incorporated with a combination of an ashless dispersant, metallic detergent and the like (Japanese Patent Publication No. 6-60317), engine oils incorporated with an oxidation inhibitor of sulfur-containing phenol derivative or the like (Japanese Laid-open Patent Application No. 6-93281), engine oils incorporated with a specific oxidation inhibitor or the like (Japanese Laid-open Patent Application No. 7-126681), and diesel engine oils incorporated with a combination of 3 types of additives (Japanese Laid-open Patent Application No. 7-207290).
Various types of base oils have been also proposed to improve properties of engine oils. These base oils include those based on mineral oil prepared to have a viscosity index of at least 80, and contain basic nitrogen at 5 ppm or less and aromatic compounds at 1% or less for the lubricant oil composition serviceable in a NO.sub.x -containing atmosphere (JP 2,564,556), those based on mineral oil or the like prepared to have a viscosity of 2 to 50 cSt at 100.degree. C. and contain aromatic compounds at 2% or less for internal combustion engine oils (Japanese Patent Publication No. 6-62988), and those based on mineral oil containing total aromatic compounds at 2 to 15 wt %, and isoparaffin and monocyclic naphthene compounds at 60 wt % or more as total content in the saturates (JP 2,724,508).
In spite of these proposals, however, no lubricant oil composition can sufficiently control poisoning of the 3-element catalytic system for occluding/reducing NO.sub.x and deposits in the air intake system in a lean combustion or direct injection engine.
It is an object of the present invention to provide a lubricant oil composition for internal combustion engines, excellent in oxidation stability in the presence of NO.sub.x and evaporation characteristics, and controlling formation of deposits in an air intake system.